Exhaust gas probes have output signals which fluctuate in dependence upon the oxygen content of the exhaust gas. It has long been known to use such probes as control sensors for controlling the mixture of an internal combustion engine. However, this is only possible if the exhaust gas probe is adequately heated because of the pronounced temperature dependency of the probe signal. The heat necessary to reach this temperature is at least partially supplied to the probe by the exhaust gases of the engine. The heat energy supplied in this manner can however be inadequate because of an unfavorable location of installation of the probe or because of the operation of the engine at a load which is too low. It has therefore been shown to be necessary to provide additional heat for such probes and to open-loop or close-loop control their temperature to obtain the most precise lambda signal possible.
Published German patent application 3,326,576 discloses that the probe, which here can have a probe ceramic having an NTC-characteristic, is directly subjected to an electrically alternating variable. The measured internal resistance of the probe ceramic is used for the temperature control of the exhaust gas probe.
A further method of heating an exhaust gas probe is for example disclosed in U.S. Pat. No. 4,294,679. Here, the exhaust gas probe is heated directly by a heater coil (PTC) mounted on the solid body electrolyte of the sensor. United States patent application Ser. No. 273,517, filed Jun. 15, 1981, discloses a method wherein a heater resistor (PTC), which is separated spatially from the exhaust gas probe, is used with an additional thermal element as a control sensor for controlling temperature.
U.S. Pat. No. 4,291,572 discloses a control of a heater of an exhaust gas probe in dependence upon the load of the engine. Furthermore, methods are also in use which utilize a deliberate increase of the exhaust gas temperature for heating the exhaust gas channel with the increase of exhaust gas temperature being caused by an intervention in the ignition and/or an intervention in the mixture. However, the above-mentioned methods are directed only to individual exhaust gas probes at least when they include control concepts. However, mixture control systems for internal combustion engines, which process the output signal of several probes, are also known. For example, U.S. Pat. No. 4,007,589 utilizes the signal of an exhaust gas probe which is mounted forward of the catalyzer as well as the signal of a second probe which is mounted rearward of the catalyzer for monitoring the catalyzer activity. The signal of the probe forward of the catalyzer is used for control.
United States patent application Ser. No. 679,050, filed May 9, 1991, discloses a method for lambda control wherein the signal of a probe mounted rearward of the catalyzer is utilized to change the actual value of a second probe utilized as a control sensor which is mounted forward of catalyzer. In addition to these methods, which include two exhaust gas probes lying one behind the other in the same exhaust gas flow, there are still further concepts for lambda control which make use of more than one probe. The so-called stereo lambda control is an example which is especially used for V-engines. Because of constructive characteristics, these engines have at least to some extent separate exhaust gas passages for the individual cylinder banks. In the context of the stereo lambda control, a separate mixture control system having its own lambda probe is provided for each cylinder bank. Since for the temperature characteristics of the exhaust gas probes which are used in multiprobe systems, the same laws apply as apply to individual systems, it is desirable also for these multiprobe systems to develop concepts for a targeted influencing of the exhaust gas temperature. As a result of such a concept, the measuring accuracy is improved with which the lambda signal is detected. A strictly open-loop control satisfies this purpose only incompletely because of its inability to respond to unanticipated disturbances. For example, disturbances in the ignition system can lead to an afterburning of the mixture in the exhaust gas channel. The temperature increase associated therewith is unnoticed by a pure open-loop control and can therefore lead to an overheating of the catalyzer and, in combination with the probe heater, lead to an undesired overheating of the probes. This disadvantage can be avoided with a temperature control loop for each individual probe. Such a solution has however the disadvantage that it is technically very complex and therefore also expensive.